EGR is a known method for reducing NOx emissions in internal combustion engines. Conventional EGR systems work by taking a by-pass stream of engine exhaust gas from an engine exhaust manifold and directing the same to an EGR valve. The EGR valve is designed and operated to provide a desired amount of exhaust gas to mix with an intake air stream and inject the mixture into the engine's induction system for subsequent combustion. The EGR valve is operated to regulate the amount of exhaust gas that is routed to the engine induction system based on engine demand.
The purpose of recirculating exhaust gas is to reduce the oxygen content of the air in the cylinder. With less oxygen to react with the nitrogen, less NOx is formed. It also lowers the temperature by absorbing some of the heat of combustion. Accordingly, such a conventional EGR system typically comprises exhaust by-pass tubing, related plumbing and manifolding, and an EGR control valve, all of which are ancillary components that are attached to the engine and/or to the area surrounding the engine.
In certain applications, is it desired that the exhaust gas exiting the EGR system and being introduced into the engine intake system for combustion be cooled for the purposes of reducing emissions, specifically NOx. Cooling the exhaust gas reduces NOx because less NOx is formed at lower temperatures. Cooler gas is also more dense so more recirculated gas can be packed into the cylinder. Accordingly, it is known that a cooler is used in certain EGR systems for the purpose of cooling or reducing the temperature of the exhaust gas that is passed through the EGR valve to the engine intake system. Typically, the EGR cooler is placed downstream from the EGR valve outlet such that all exhaust gas exiting the valve for directing to the engine intake is routed through the cooler. Such EGR coolers can be air or water cooled, and can be configured having single or multiple passes, as required for the particular application.
An issue that is known to exist with such conventional EGR systems comprising a cooler is that, under certain operating conditions, the temperature within the cooler can cause the cooling medium (typically in liquid form) that passes therethrough to reach its bubble point and boil. When the cooling medium first reaches the boiling point, nucleate boiling is induced, where very small bubbles form and collapse. This is a beneficial phenomenon and actually improves the heat transfer. However, only a small increase in heat load (or a reduction in coolant pressure) can shift the boiling mode to transition boiling, which is an unstable mode that can lead to large bubbles that leave vapor on the heat transfer surface. This phase change from liquid to vapor causes two undesired events to occur. First, the cooling medium no longer performs its cooling function, thus fails to reduce the temperature of the hot-side liquid or gas being passed through the cooler. Second, the phase change from liquid to vapor that occurs at the during transition boiling causes the pressure within the cold-side of the cooler to increase, thereby operating to potentially damage the cooler itself and/or other devices that are connected to the cooler.
It is, therefore, desired that a cooling system as used with internal combustion engines, e.g., as used within an EGR cooling system, be configured and/or operated in a manner that can help reduce the occurrence of cooling medium boiling, which may occur during infrequent episodes of high gas temperatures or during situations where the coolant temperature may increase due to high ambient temperature conditions, thereby increasing the effective service life of such cooling system. It is desired that such cooling system and/or method for controlling the same be relatively easy to implement and not take up excessive space in the engine compartment. It is further desired that such cooling system be configured in a manner capable of being incorporated into an engine EGR system an engine without unnecessary complexity.